You can see the most important sights also of the Pest and the Buda side during this 8 hour long tour. This time is enough to explore the city more deeply and get a lot of information about Hungarian history, culture, arts, politics, gastronomy, people - and about anything what you want.
Sightseeing by car is more comfortable and less tiring - and allows more time to spend in all places.
If you are more in number than 3, I can arrange a minivan.